#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define endl '\n'
const int N = 3e5 + 5;

int n, m;
int a[N];
ll sum[N];

int main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  cin >> n >> m;
  rep(i, 1, n) cin >> a[i];
  rep(i, 1, n) sum[i] = sum[i - 1] + a[i];
  deque<int> dq;
  ll ans = -1e18;
  rep(i, 1, n) {
    int j = i - 1;
    while (dq.size() && sum[dq.back()] >= sum[j]) dq.pop_back();
    dq.push_back(j);
    int k = i - m;
    while (dq.size() && dq.front() < k) dq.pop_front();
    ans = max(ans, sum[i] - sum[dq.front()]);
  }
  cout << ans;
  return 0;
}